1. Field of the Invention
The invention relates to a process for the preparation of alkali metal dichormates and chromic acid by electrolysis of alkali metal monochromate and/or alkali metal dichromate solutions in electrolysis cells, the anode and cathode compartments of which are separated by cation exchange membranes.
2. Description of Related Art
According to U.S. Pat. No. 3,305,463 and CA-A-739,447, the electrolytic preparation of alkali metal dichromates and chromic acid is carried out in electrolysis cells, the electrode compartments of which are separated by cationic exchange membranes. In the production of sodium dichromate, sodium monochromate solutions or suspensions are passed into the anode compartment of the cell and converted into a sodium dichromate solution by selectively transferring sodium ions through the membrane into the cathode compartment. For the preparation of chromic acid, sodium dichromate or sodium monochromate or a mixture of sodium dichromate and sodium monochromate is passed into the anode compartment and converted into the solution containing chromic acid. In both processes, an aqueous solution of sodium hydroxide is obtained in the cathode compartment.
Membranes which are sufficiently chemically, thermally and mechanically stable and based on perfluorinated polymers having exchanger groups are preferably used as cation exchange membranes in the stated processes. These membranes may have both a single-layer structure and a two-layer structure, the two-layer membranes as a rule more effectively suppressing the diffusion of hydroxide ions through the membrane, which leads to a higher current efficiency of the electrolysis. However, this improved current efficiency is generally associated with a higher cell voltage than that achieved with the use of single-layer membranes.
Such cation exchange membranes are described in, for example, H. Simmrock, E. Griesenbeck, J. Jorissen and R. Rodermund, Chemie-Ing. Techn. 53 (1981), No. 1, pages 10 to 25 and are commercially available, for example, under the name Nafion.sup.R (manufacturer: E. I. DuPont De Nemours & Co., Wilmington, Del./USA).
In addition to the lower cell voltage achievable, single-layer membranes have the advantage that, compared with two-layer membranes, they are less sensitive to polyvalent cations, in particular calcium ions and strontium ions, in the alkali metal chromate and/or alkali metal dichromate solutions, which lead to precipitation of polyvalent cation compounds in the membrane and consequently to a deterioration in the functioning of the membrane.
The object of the invention is to provide a process for the preparation of alkali metal dichromates and chromic acid, which process does not have the disadvantages described.